Voice order through Alexa only - by saying "Alexa, order an Amazon Smart Plug". By far the lowest price I've seen - it makes the Best Buy $5 deal earlier look expensive.
One per account, as far as I can tell.
From the promotional email:
"In order to qualify for the Amazon Smart Plug offer discount offer, you must: (1) have activated your first Amazon Echo device between 12/18/2019 12:00 a.m. (PT) and 12/31/2019 11:59 p.m. (PT), and (2) ask Alexa to order the Amazon Smart Plug using Alexa on the same device. Alexa will either add the Amazon Smart Plug to your Amazon shopping cart or prompt you to complete the purchase on Alexa. Offer expires at 11:59 p.m. on 1/31/2020 (PT). Only first time device owners of any Amazon Echo device are eligible for this promotion. Shipping charges and taxes may apply to the full value of discounted and free promotional items. Offer limited to one per customer and account. The maximum benefit you may receive from this offer is $24. Promotion only applies to Amazon Smart Plug. Discount automatically applied at checkout. Amazon reserves the right to modify or cancel the offer at any time. Offer is non-transferable and may not be resold. If any of the products or content related to this offer are returned, your refund will equal the amount you paid for the product or content, subject to applicable refund policies. If you violate any of these terms, the offer will be invalid. Offer only applies to orders using Alexa Shopping on an Amazon Echo device. Offer not available using Alexa in the Amazon app. While supplies last."